Experimental method and process control of capillary flow focusing

Capillary flow focusing as one typical kind of capillary flows is able to produce droplets and capsules at micro-scales, offering promising advantages in applications. This work reviews the experimental methods of capillary flow focusing based on the 'air-injecting' and 'air-sucking' devices, and presents the complete platform for studying the fluid cone-jet structures. Moreover, the method of adding external actuations on the flow focusing device to manipulate the breakup of liquid jets is illustrated. The flow visualization methods to capture the interface evolutions of the compound fluid cones and jets are introduced. It is found that the geometric parameters and the external flow parameters significantly affect the morphologies and instabilities of the fluid cone, and would further affect the diameters, perturbation wavelengths and interface coupling of the coaxial liquid jets. The lens effect of the outer interface on the coaxial jets could cause distortion of the inner jet and droplets, and a modified method was developed to remove the distortion based on the law of refraction. For the jet breakup upon external actuation, the relationship between the jet breakup length and the excitation amplitude was studied. The effects of the actuation frequency on the diameters and monodispersity of resultant droplets were studied.